Ensure map legend labels correspond to visible shaded areas.

Fix a bug where, due to records overriding one another, a legend
label could appear on the map without any corresponding shaded
areas. One example of this was the New England map for Sambucus
nigra showing the non-native legend label even though the non-
native shaded areas had been overridden by 'absent' records. (#558)
Correct shading of infraspecific taxa on maps. (#558)